Aircraft hydraulic systems supply numerous hydraulic loads that are critical for flight control and safety. In general, aircraft hydraulic systems supply two major hydraulic load centers--airframe hydraulic loads and propulsion control hydraulic loads. Airframe hydraulic loads include loads such as: the nose landing gear extension and retraction; the main landing gear extension and retraction; the main landing gear brakes; the high lift devices on the wing leading edge and trailing edge; the ailerons; the flaperons; the spoilers; the rudder; the horizontal stabilizer; and the elevators. Propulsion control hydraulic loads include loads such as: the thrust reverser; the engine inlet bypass door; the engine variable geometry inlet; and the engine variable geometry nozzle. In order to reduce maintenance requirements and weight constraints, it is well known to integrate propulsion hydraulics with airframe hydraulics by supplying propulsion control hydraulic loads and airframe hydraulic loads with the same engine-driven hydraulic pump. Accordingly, integrated hydraulic systems are desirable in supersonic aircraft.
Casualty control considerations are major factors in the design of integrated aircraft hydraulic systems. For example, in the event of an engine fire, it is important to stop the supply of hydraulic fluid into any compartment where ignition sources are present, such as to an engine mounted hydraulic pump. To that end, known systems are configured for preventing the flow of hydraulic fluid from an aircraft hydraulic system from feeding an engine fire. Such a known system includes a firewall shutoff valve located in the hydraulic pump suction line. The shutoff valve is normally open, but can be closed to prevent hydraulic fluid from the hydraulic system from feeding an engine fire. Another known system includes a reservoir level sensor, and first and second switches at first and second reservoir levels. When the reservoir level sensor senses a drop in reservoir level to the first level, the first switch actuates and isolates one-half of the hydraulic loads in the hydraulic system. If the reservoir level continues to drop to the second level, the second switch actuates. This restores to service the hydraulic loads previously isolated, and isolates the remaining half of the hydraulic loads of the hydraulic system. Such a system does not maintain a continuous supply of hydraulic fluid to propulsion control hydraulic loads.
It is important to maintain a continuous supply of hydraulic fluid to propulsion control hydraulic loads in the event of a hydraulic system failure. In known integrated hydraulic systems, a failure, such as a leak in the airframe hydraulic loads, may be controlled by closing the firewall shutoff valve. This results in an unnecessary loss of propulsion control hydraulics. Further, failures within a hydraulic system that cause leaks result in loss of operation of the entire hydraulic system. A majority of these failures occur in components outside the engine-installed hydraulic equipment.
The loss of propulsion control hydraulics especially impacts engine control in supersonic aircraft. Typically, supersonic aircraft engines include two sets of engine inlet controls. Each set of engine inlet controls is generally supplied by a separate engine-driven hydraulic pump. The engine inlet controls are necessary for containing the sonic shock wave generated during supersonic operation within the engine inlet, and preferably maintaining a normal shock wave position within the engine inlet. If both sets of engine inlet controls are lost due to a loss of the propulsion control hydraulic systems, the normal shock wave cannot be contained within and maintained at the proper position within the engine inlet. The engine must then be operated in a transonic or subsonic mode. Therefore, in a supersonic aircraft having an integrated hydraulic system, it is desirable to maintain hydraulic power to both sets of engine inlet controls for controlling the engine.
A dedicated propulsion control hydraulic system that is separate from an airframe control hydraulic system can be provided to maintain continuity of propulsion control hydraulics in the event of a failure of airframe control hydraulics. However, a dedicated propulsion control hydraulic system greatly increases maintenance requirements and adds considerable weight. Therefore, it would be desirable to provide an integrated aircraft hydraulic system with a system for maintaining a continuous supply of hydraulic power to propulsion control hydraulic loads, including engine inlet controls, in the event of a failure in the airframe hydraulic loads. However, no current integrated aircraft hydraulic systems are known to provide a system for maintaining continuity of hydraulic power to propulsion control hydraulic loads.